This invention relates to processing polyimides.
Polyimides are currently used in high temperature composite and adhesive applications. One well-known polyimide is Polymerization of Monomer Reactants ("PMR")-15, a low molecular weight (c.a. 1500), norbornene-terminated polyimide prepared by reacting benzophenone tetracarboxylic acid anhydride and methylene dianiline. These polyimides are typically supplied to the end user in the form of a prepreg fabric or scrim cloth impregnated with the polyamic acid precursor of the polyimide. Upon thermal processing, the polyamic acid groups are converted to imide groups to form a fully imidized polymer, while crosslinking takes place simultaneously through polymerizable end groups (e.g., norbornene groups in the case of PMR-15).
These polyimides suffer from two problems which limit their utility. First, conversion of the polyamic acid groups to imide groups during thermal processing releases large amounts of volatile material. This leads to voids in the final polyimide, resulting in a weakened product. Second, the viscosity of the polyimides prior to thermal processing is too high to permit easy processing. The problem is particularly acute in the case of polyimide adhesives, which have too great a viscosity to be reticulated with a hot air knife.